Answer:

Triangle sum theorem states that all angles in a triangle must add up to #180^@#, a similar theorem applies to quadrilaterals and it states that all angles in a quad. must add up to #360^@#.

Explanation:

You have already applied the triangle sum theorem which states that all 3 angles in a triangle add up to #180# degrees it seems, so now all you have to do is create an algebraic expression that reflects that.